I was sitting on a bench on a dimly lighted subway platform, waiting for the B train, when the announcement came. It was startling, first of all because the words were clear - the first time I had ever been able to identify a syllable muttered through the PA system of the New York City subways.

The mysteriously discernible message asked travelers to be alert and report any "suspicious" person or behavior. A gray-haired woman next to me unleashed an expletive in a delivery worthy of a New Yorker. I smiled my approval. The other six people waiting for the train inched down the platform uncomfortably, and it occurred to me that the outspoken woman and I were being regarded as suspicious.

Similar announcements are made at train stations and airports and written on posters across the United States. Right after the Sept. 11, 2001, attacks, we were supposed to be on the lookout for terrorists.

But rapidly the U.S. government has turned us into the arbitrators of a more general condition known as "suspiciousness." This is problematic.

Remember how, just after the attacks, Sikhs in this country were singled out by angry, frightened citizens as suspicious because they wore turbans, as did Afghanistan's Taliban rulers? It is a tragic waste of historical experience that no one today wants to study the fall of communism. Communism was not defeated by the Cold War; it collapsed incrementally from hundreds of bad decisions. And one of those decisions was setting neighbor against neighbor, watching for suspicious activity.

After the Russian Revolution in 1917, with half the world plotting to overthrow the new Soviet government, Soviet security asked the people to be on the lookout for conspirators. Then, as now, there were probably some good reasons to be vigilant. But those officials also used an unnecessarily vague term - "counterrevolutionary activity." The initial implication was there were people who were working with foreigners to overthrow the revolution. But quickly "counterrevolutionary" became anything that was not in lock step with the government. The government wanted everyone reporting on everyone, with the government deciding how to evaluate the information. Casual social encounters, play dates between children, conversations on banal subjects were reported and filed.

At the outset of the Cold War, East Germany also had reason to be vigilant. It was laced with Western agents seeking to sabotage the country. Some of them were Germans with Nazi pasts. The East German security force, the Stasi, asked citizens to inform on any activity that seemed suspicious. Many did so because they felt their country was being threatened. Eventually this settled into an informant network in which the mundane activities of a large part of the East German population was being reported to the Stasi.

CIA sabotage, assassination attempts and other mayhem in Cuba during the 1960s are well documented. The Cuban government responded with the Committees for the Defense of the Revolution. By 1965, one out of every two Cuban adults belonged, and most Cubans informed on their neighbors. What was considered counterrevolutionary activity? It could be almost anything, including an expletive hurled at a government announcement. In the years since, Cubans have learned to distrust their friends and neighbors.

This practice of turning as many people as possible into government snitches rotted these societies from within, and turned civic-mindedness into a base instinct rather than a lofty one. After the fall of the Soviet Union, the most despised people from the old regime were not the officials, not even the secret police officials; they were the informants.

In 1993, I was in Berlin, having dinner with some former citizens of the communist state whom I knew. They were idealists who had loathed the materialism of West Germany and had believed in their old state. They couldn't work anymore and had become pariahs because they had been identified as people who had informed to the Stasi.

They complained of their plight and explained that they had simply been defending their country. They did not see the role they had played in its slow dismantling. One of them asked me, "If you were told that your country was threatened, wouldn't you give information to your government to help it?"

I thought about this because I felt that they deserved the most honest answer I could give. But it was very difficult to relate my experience in the United States to theirs in East Germany.

I thought of the FBI informants who infiltrated the peace movement in the 1960s, and the former communists who had named names to congressional committees a decade earlier. The problem was that every example I could think of was not a true threat. Finally, I had to say, "No, I wouldn't."

But today the U.S. government asks for help with what I believe is a real threat. But I still have to say, "No, I wouldn't." A system of informants undermines a society and is inevitably abused by government. Governments too easily become addicted to information. And that is how it always begins.

Mark Kurlansky is the author, most recently, of "1968: The Year That Rocked the World."